The role of phosphorylase kinase in the nervous and hormonal control of glycogenolysis in muscle

The total dependence of phosphorylase kinase activity on calcium ions synchronizes glycogenolysis with the onset of muscle contraction. The activity is also stimulated by phosphorylation of the enzyme, a reaction catalysed by cyclic AMP dependent protein kinase and also by phosphorylase itself. Phosphorylase kinase possesses the structure (αβγ)₄, where the mol wts of the three protein subunits α, β and γ are 145,000, 130,000 and 45,000 respectively. The 40 fold activation produced by cyclic AMP dependent protein kinase is paralleled by the phosphorylation at a single site on the β subunit, although one site on the α subunit is also phosphorylated at 5 fold slower rate, without affecting the total activity. The phosphorylation of the α subunit does, however, greatly stimulate the dephosphorylation of the β subunit and inactivation of the enzyme by phosphorylase kinase phosphatase kinase may also be activated up to 100 fold by limited proteolysis, but in contrast with activation by phosphorylation this appears to correlate with a modification of the α sununit. Models for the interrelationship between the various activating mechanisms are considered, and the roles of the three sununits are discussed in terms of electron micrographs of the enzyme molecule.